Piston pins and crankpins of a reciprocating internal combustion engine may be linked by a compound link type piston crank mechanism. The compound link type piston crank mechanism generally has an upper link linked to the piston pins of the pistons, a lower link that links the upper link and the crankshaft crankpins, and a control link, one end of which is supported such that it can rock on the engine, and the other end of which is linked to the lower link. The upper link and the lower link are rotatably linked or coupled to each other through an upper pin, and the control link and the lower link are rotatably linked or coupled to each other through a control pin.
The lower link in the compound link type piston crank mechanism receives the combustion pressure to which the pistons are subjected through the upper link, and transmits the force to the crankpins by a “lever” type action using the control pins as fulcrums.
The maximum combustion gas force to which a lower link is subjected by a piston is input from the upper pin bearing through the piston pins, the upper link, and the upper pins. The load and inertial forces are counterbalanced by a load that is also generated in the crankpin journal and control pin bearing.
The load received from the upper pins is often off center or tilted and can be directed toward the crankpins, such that offset deformation of the upper pin bearing may be leveraged by the length of the upper link and adversely affects the crankpin journal.